You could use the NetFileClose api function for this. This would force the resource to be freed. Please note that this function just kicks the user out of the app, without proper closing it. The memory of the user's machine would not be properly cleared.
That is the reason I didn't include it in the application. Instead I send a network message to the user's machine and hope they will listen.

PS, you can always walk to the server, open control panel --> server and kick them out there.